Multi-tracer logging technique

ABSTRACT

Method of logging a plurality of underground formations wherein a first and a second gamma ray emitting radioactive tracers with different energy levels are introduced into the formations. Logs are produced of the tracers as the traced fluids pass through the formations and thereafter the logs are analyzed to determined changes in effective permeabilities and sweep of the formations.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improved method of determining sweepand effective permeabilities of fluids flowing in an undergroundformation and, more particularly, to such a method which utilizes one ormore gamma ray emitting tracers which are logged by the same loggingtool.

2. Setting of the Invention

During a secondary or tertiary oil recovery project a displacement fluidor fluids are injected into the formation through an injection well andoil in the formation is displaced towards an offset production well. Theefficiency with which the injected fluid contacts the oil bearing zonesis termed "sweep efficiency". The percentage of available oil displacedby the injected fluid is termed "displacement efficiency". Bothparameters, "sweep efficiency" and "displacement efficiency", areimportant in evaluating the displacement process and in obtaining adescription of the reservoir. Various individual methods have beenutilized to determine sweep efficiency of a waterflood project, andvarious individual methods have been utilized to determine the effectivepermeabilities of the swept formation. One such method used in the pastinvolves injecting a first tracer fluid into the formation along withthe injection fluid and at some later time injecting a second tracerfluid into the formation. A logging device run through an observationwell would indicate the presence of the tracer fluids as they passthrough the formation. However, experience has shown that many monthsafter breakthrough of a tracer into a particular zone, a high backgroundradiation of the first tracer may still be present. Thus, it has beenextremely difficult to distinguish between the old tracer and the new,although an approximate evaluation could be made. If this evaluation wasattempted with tracers injected several weeks apart, it would bevirtually impossible to distinguish between the tracers at all. Thus,there is a need for a method of utilizing at least two radioactivetracers which can be distinguished.

SUMMARY OF THE INVENTION

Disclosed is a novel method of logging a plurality of undergroundintervals within a given formation, contemplated to overcome theforegoing disadvantages. A first radioactive tracer is introduced intothe formation and the first tracer is logged as it passes through theformation. A second radioactive tracer is introduced in the sameformation, and the second tracer is logged as it passes through theformation. The second tracer has a different energy level than the firstlevel. The logs are then analyzed to determine changes in the effectivepermeabilities of the formations.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a sectional view of a plurality of wells penetrating aplurality of underground formations and which illustrates theadvancement of tracer fluids through the formations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in the drawing, reference character 10 indicates an injectionwell which penetrates a plurality of underground oil bearing intervalsof a given formation or separate formations, 12, 14, and 16. The well 10is comprised of a casing 18 and an inner tubing 20. A plurality ofperforations 22 extend through the casing 18 and are spaced adjacent theformations 12, 14, and 16. The well 10 is completed in a conventionalmatter as is well known in the art.

The tubing 20 is set through a packer 21 above the formations ofinterest 12, 14, and 16, to allow any radioactive fluids to be displacedfrom the well 10, as will be described hereinbelow. Water is injectedthrough the tubing 20 and into the formation 12, 14, and 16 as in awaterflood project to drive in-place oil and other hydrocarbons towardsa production well 24. The produced oil from the production well 24 iscollected at the surface for use elsewhere. Interposed between theinjection well 10 and the production well 24 is an observation well 26.The observation well 26 is completed utilizing a nonperforated casing 28which can be metallic or nonmetallic, such as fiber glass or plastic.

In order to determine vertical sweep and effective permeabilities of theformations 12, 14, and 16, the following procedure is utilized inaccordance with the present invention. A first radioactive tracer isinjected through the injection well 10 into the formations 12, 14, and16. The tracers utilized in this invention produce gamma radiation. Suchtracers which can be utilized include such water soluble tracers ascobalt 60, cobalt 58, cobalt 57, or strontium 85, iodine 131, and also agas, such as Krypton 85. A predetermined period of time later, fromseveral days to several weeks, a logging tool 30, capable ofmulti-channel analysis of gamma ray energy, such as a device marketedunder the trademark "Spectralog" developed and utilized byDresser-Atlas, is lowered into the observation well 26. Logs from thelogging tool 30 obtained over a period of days record the advancementand energy levels of the tracer fluid as it passes through theformations. The waterflood project is continued and after apredetermined period of time, usually from several weeks to severalmonths, a second radioactive tracer is then injected through theinjection well 10 into the formations. The second tracer has aradioactive energy different than that from the first tracer fluid.Again a log is made utilizing the logging tool 30 through theobservation well 26 as the second tracer fluid passes through theformations.

As can be seen from the drawing, the advancement of the second tracerfluid through the formation over the same given period of time iseffectively greater than that of the first tracer due to the fact thatthe permeability of the intervals are different as the fluid saturationwithin the formations changes as the flood project progresses. As shownin the drawing and specifically referring to formation 14, the secondtracer over the same period of time has traveled a greater distance thanfirst tracer fluid and effectively has progressed much further than thefluids through the other formations 12 and 16.

After the second log is produced, the two logs are compared so that theeffective permeabilities in the formations and the changes therein maybe determined. Various methods for comparing the logs may be utilizedincluding graphical, analytical, and digital analysis, as by utilizing acomputer. From this data, a determination may be made of (1) the zoneswhich are invaded by the injected fluid or fluids and the verticalextent to which these zones are swept; (2) the continuity of the "payzones" between the injection, observation and producing wells in thesame flood patterns; (3) the effective permeability of the floodedzones; (4) the relative vertical sweep efficiency of the varioussequentially injected fluids; and (5) the evaluation of sweepimprovement treatments or other enhanced oil recovery processes.

The above information is obtained through the observation well 26 in theinterwell area and thus should be more representative of the floodpattern than the same information obtained at a production well orinjection well which may have been subjected to various stimulationtreatments.

The described method may be utilized in a five-spot or multi-wellinjection program wherein a different tracer is injected into eachseparate injection well, and these tracers are logged through anobservation or observation wells adjacent the production well.Alternately, a different energy tracer may be injected into eachseparate well and logged which would not only indicate sweep efficiencyand permeability change, but also whether one zone or formation is incommunication with another.

The above technique lends itself readily to evaluation of several areasof enhanced recovery where methods of tracing and tracking the relativerates of advancement of injected fluids is needed. Areas that arevisualized for using this method of evaluation are water-gas-water sluginjections to determine the relative rates of fluid advancement,presence of fingering, which zones are being swept, whether both fluidssweep the same zone and/or influence of different fluids on effectivepermeabilities. The above method may be used in preflushwater-micellar-polymer injection banks to determine relative rates ofadvance and which zones are being swept. Also, the above method may beutilized in sweep efficiency tests where a polymer, foam or othersimilar materials used for sweep control are injected.

The radioactive tracers may be utilized to accompany injected floodingfluid or sweep control fluid to determine which zones are being swept,and whether the same zones are still being swept, relative rates ofadvance and/or effective permeabilities of flooding fluids before andafter sweep control has been instigated. The above method may also beutilized in stimulation tests where the injection fluids are tracedbefore and after stimulation to determine which zones are being swept,whether the same zones are being swept, relative rates of advance,and/or effective permeabilities to injected fluids before and afterstimulation.

The above method may also be utilized without the requirement of anobservation well. The requirement for an observation well to log throughis necessitated by the fact that a wellbore to be utilized must beisolated from the reservoir so that any gamma ray increase overpreviously measured background ratings will indicate the passage oftrace fluids past the well in that zone. If the casing was perforated orthe well was not cased, the trace fluids could enter the wellbore anddiffuse with the wellbore fluids, thereby smearing the identification ofthe intervals. Cross flow of fluids from one interval to another by flowthrough the wellbore could also be misinterpreted as an interval invadedby interwell flow. Therefore, to eliminate these wellbore effects, ithas been necessary to use cased observation wells for this type ofmonitor logging. Preferably, the observation well would be located inthe interwell area between an injection and a producing well.

One method of logging through a production well which could eliminatethe requirement of logging through an observation well is accomplishedby replacing the wellbore fluids in the production well with anonradioactive water. This procedure involves producing the productionwell to establish representative radioactive tracer concentrations inthe formation in the vicinity of the well, obtaining a base log of thewell while producing, and injecting water down the annulus (between thecasing and the tubing with the bottom of the tubing set below theformations of interest) to displace the radioactive traced water out ofthe hole while continuing the production of the well. The injection rateof water into the annulus is varied until the well hydrostatic pressurejust balances the formation pressure. The injection-production procedureof the well is continued until all the radioactive fluids have beenproduced from the well. While this injection-production continues, a logof the well is obtained utilizing a gamma ray device capable ofmultichannel analysis and is compared to the base gamma ray log todetermine the intervals containing the different gamma ray tracematerials. Next the injection of the water into the annulus is stopped,but the production of fluids is continued. A log is then obtained andincreases in log radioactivity due to fluids being produced into thewell should better define the tops of the productive intervals. Theproduction of the well should then be shut down with continued loggingof the well. This would aid in defining the bottom of the productionintervals.

Whereas the present invention has been described in particular relationto the drawing attached hereto, it should be understood that other andfurther modifications, apart from those shown and suggested herein, maybe made within the scope and spirit of this invention.

We claim:
 1. A method of determining flood sweep efficiency andeffective permeability changes in a formation caused by the flood bylogging at least one radioactive tracer in an underground formationcontaining fluids and penetrated by an injection well and a productionwell, which has a casing and an annular space between the casing andwellbore, comprising in sequence:(a) injecting a tracer fluid throughthe injection well into the formation; (b) producing fluids through thecasing of the production well to establish a representative tracerconcentration within the formation adjacent to the production well; (c)obtaining a first tracer log through the casing of the production well;(d) injecting a fluid under pressure into the annular space whileproducing fluids through the casing of the production well to displaceany tracer present in the production well; (e) injecting the fluid at agreater pressure than in step (d) to equalize fluid pressures in theformation; and (f) obtaining a second tracer log through the casing ofthe production well, whereby the first and the second tracer logs arecomparably analyzed to determine the flood sweep efficiency and theeffective permeability changes in the formation.
 2. The method of claim1 and including after step (f) the following steps in sequence:(g)ceasing the injection of the fluid; (h) obtaining a third tracer logthrough the casing of the production well; (i) ceasing the production offluids through the casing of the production well; and (j) obtaining afourth tracer log through the casing of the production well, whereby thethird log indicates the tracer concentration adjacent an upper portionof the formation and the fourth log indicates a tracer concentrationadjacent a lower portion of the formation.